Washington: Louisiana police have shot and killed a black man selling CDs outside a convenience store, triggering protests over the latest in a string of cases of alleged police brutality against black suspects.

A video shot by a witness and circulated online, showed two officers ordering a tall black man to the ground early Tuesday.

As he remained standing, the officers wrestled him onto the roof of a car and then to the ground. While he was pinned down by the officers, one pulled a gun and shot him four times at close range.

"They shot him? Oh my fucking goodness!" a witness can be heard as exclaiming in the background of the video.

At least one woman can be heard wailing and repeating "Oh my God!"

Police in the state's capital Baton Rouge identified the man as Alton Sterling, 37, saying two officers "made contact" with him in the parking lot of the convenience store.

The shooting took place at 12.35 am (0435 GMT) Tuesday after police received a tip from an anonymous caller who said they had been threatened by a man with a gun, according to a police statement.

"An altercation between Sterling and the officers ensued. Sterling was shot during the altercation and died at the scene," police said on Facebook.

Family and friends of Alton Sterling protest in Louisiana after Sterling was fatally shot in an altercation with Baton Rouge Police. AP

They said that in accordance with "standard procedure," the two officers involved in the shooting had been placed on administrative leave during the ongoing investigation.

The East Baton Rouge Parish Coroner's Office said Sterling was killed by multiple gunshot wounds to the chest and back, WAFB television reported.

News reports said the officers were wearing body cameras, which police said fell off during the confrontation.

However, in addition to the witness video, the shooting was captured by the store's surveillance camera and at least one patrol car dashboard camera, state representative C. Denise Marcelle told news media after being briefed by the police chief.

About 100 people, including friends and relatives of Sterling, protested outside the convenience store after shooting, some blocking roads, The Advocate reported.

"What I said to the (police) chief is he has to have transparency in this matter because as you can see, this is getting out of hand. People are clearly upset and they want transparency," Marcelle said.

Edmond Jordan, an attorney representing Sterling's family, said the shooting was completely unjustified.

"That police officer fired two shots and he seemed to pause for a while and shot him two more times. I don't think any of those shots were justified, but I'm curious as to why he paused for such a long time and then fired two more times," he told CNN.

Jordan, who is also a state legislator, added that Sterling was selling CDs with the permission of the convenience store owner.

Triple S store owner Abdullah Muflahi told The Advocate that he witnessed police retrieving a gun from Sterling's pocket after the shooting, although Sterling did not hold a weapon during the altercation.

"They were really aggressive with him from the start," Muflahi said about the police officers, adding that they were "freaking out" after the incident.

Muflahi said he heard one of the officers say: "Just leave him."

Civil rights group Black Lives Matter, which was referenced by many protesters in Louisiana, tweeted: "Enough is enough. #AltonSterling."

African American civil rights leader Jesse Jackson also expressed outrage over the shooting on Wednesday.

"The shooting of #AltonSterling in #BatonRouge is a legal lynching. Justice must prevail. #Outraged," Jackson wrote on Twitter.